1. Field of the Invention
The present invention relates in general to open-type scroll compressors, and relates in particular to an open-type scroll compressor that can be operated in the critical region of a cooling medium such as carbon dioxide to provide vapor compression cooling cycles.
2. Description of the Background Art
From the standpoint of environmental protection, there has been proposals, in recent years, to use carbon dioxide gas as a replacement for freon gas as a working gas (cooling medium) to provide cooling cycles (referred to as CO.sub.2 cooling cycles), for example, in a Japanese Patent Application, First Publication Hei 7-18602. The operation of the CO.sub.2 -based cooling cycle is similar to that of the conventional vapor compression cooling cycle based on freon. That is, as shown in FIG. 7 (using a Mollier diagram for CO.sub.2) by an A-B-C-D-A cycle, the compressor compresses gas-phase CO.sub.2 (A-B), and the compressed high temperature gaseous CO.sub.2 is cooled in a heat dissipater (gas cooler) (B-C). Next, the gas pressure is reduced (C-D) in a pressure reducer, and the condensed liquid-phase CO.sub.2 is vaporized (D-A) so that the latent heat of vaporization is gained from an external fluid medium such as air thus resulting in cooling the external fluid.
However, because the critical temperature for CO.sub.2 is 31.degree. C. which is lower than that of freon, which is the conventional cooling medium, so that when the outside temperature is high, such as during the summer season, the temperature of CO.sub.2 in the heat dissipater circuit becomes higher than the critical temperature of CO.sub.2. In other words, CO.sub.2 does not condense at the exit-side of the heat dissipater (line BC does not cross saturated liquid line SL). Also, because the conditions at the exit-side (point C) of the heat dissipater are determined by the discharge pressure of the compressor and the temperature of CO.sub.2 at the exit-side of the heat dissipater and the temperature of CO.sub.2 at the exit-side of the heat dissipater is determined by the heat releasing capability of the heat dissipater and the outside temperature (not controllable), the temperature at the exit-side of the heat dissipater cannot be controlled in practice. Therefore, it follows that it is possible to control the conditions at the exit-side (point C) of the heat dissipater by controlling the discharge pressure of the compressor (heat dissipater exit-side pressure). In other words, to obtain sufficient cooling capacity (enthalpy difference) when the external temperature is high such as during the summer season, it is necessary to increase the heat dissipater exit-side pressure as shown by a cycle E-F-G-H-E. For this reason, it is necessary to increase the operating pressure of the compressor for CO.sub.2 -based cooling cycle compared with that for conventional freon-based cooling cycle.
For example, in an automobile air conditioner, operating pressure required for conventional R134-based (freon-based) compressor is about 3 kg/cm.sup.2 while it is 40 kg/cm.sup.2 for CO.sub.2 based compressor, and the stationary pressure is about 15 kg/cm.sup.2 for R134 (freon) while that for CO.sub.2 is 100 kg/cm.sup.2. Therefore, it is necessary for the compressor to be built to withstand the pressure of such a high magnitude.
An example of the compressor used in the conventional automobile air conditioner is shown in FIG. 8. As shown in this diagram, a spiraling scroll 52 is provided inside a housing 51, and a fixed scroll 53 for engaging with the spiraling scroll 52 is situated above the spiraling scroll 52.
Inside a cylindrical boss 54 formed in the center section of the outer surface (lower surface in the diagram) of the end plate 52a of the spiraling scroll 52, an eccentric shaft 55 is freely rotatably supported by a scroll bearing 56, which also serves as the radial bearing. The eccentric shaft 55 is able to rotate eccentrically with a radius p by means of an eccentric drive, which is omitted from the diagram.
Also, between the outer surface periphery of the end plate 52a and the fixed frame 57 fixed to the housing 51, a thrust ball bearing 58 is provided to support the spiraling scroll 52.
This thrust ball bearing 58 is comprised by a pair of ring shaped race members 59 mounted on the fixed frame 57 and the spiraling scroll 52 and balls 60 intervening between the race members 59. On opposing surfaces of the pair of race members 59, spiraling race grooves 61 are disposed in several places for providing rolling motion of the balls 60. These race grooves 61 are formed in an arc shape such that the profile radius of the groove is slightly larger than that of the balls 60.
The operation of the thrust bearing so constructed will be explained below. The spiraling scroll 52 is driven by the eccentric shaft 55 to produce spiral revolution with a scroll radius .rho.. During the motion, the fixed frame 57 is coupled to the spiraling scroll 52 by means of the balls 60 intervening between the race members 59, and, because the rolling range of the balls 60 is restricted by the race grooves 61, the spiraling scroll 52 is prevented from self-rotating about its own axis.
Also, a large axial load is applied to the spiraling scroll 52 by the pressure from the compressed gas, but the axial load is supported by the balls 60 and the race members 59.
The thrust ball bearing 58 described above not only supports the load in the thrust direction but also prevents self-rotation of the spiraling scroll 52.
In other words, because the fixed frame 57 and the spiraling scroll 52 are coupled by means of the balls 60, the race grooves 61 of the race members 59 on the fixed frame side slide against the balls 60, and the race grooves 61 of the race members 59 on the spiraling scroll side slide against the balls 60.
Specifically, as shown in FIG. 9, while the ball 60 is under the load Ft in the axial direction generated by the compressed gas acting, it is also under a pressing force Fh acting in the left/right direction resulting from the tendency of the spiraling scroll 52 to self-rotate about its own axis. The ball 60 exerts a reaction force to this pressing force Fh to prevent self-rotation of the spiraling scroll 52.
However, because the ball 60 is rolling on the race groove 61, the pressing force Fh acting in the left/right direction causes the ball 60 to slide against the race groove 61, thereby generating friction at the interface. For this reason, lubrication film between the ball 60 and the race groove 61 is lost and the mechanical loss is increased, and the ball 60 and race groove 61 are worn by the friction to lead to shortening the service life of the bearing.
Frictional effects become significantly higher the higher the load on the bearing. This effect becomes particularly severe when CO.sub.2 is used as the working gas because the compressed gas pressure is higher compared with freon gas, and presents a problem that the bearing service life is reduced considerably.